1. Field of the Invention
This invention relates generally to the field of intermittent linear motors for use in combustion gas powered tools such as those used to drive fasteners.
2. Description of Related Art
The cycle of the intermittent linear motor is different from that of a continuous running engine. It does not continue automatically, as would be the case in a reciprocating internal combustion engine. Instead, the intermittent linear motor's power piston must be returned to, and remain in, a starting or rest position between each power stroke. Typically, a rod fitted to the power piston engages a fastener or other load and mechanical energy is transmitted through the rod to drive a fastener or perform other useful work during the power stroke.
The power piston is returned to its starting or rest position within a piston cylinder during a reciprocation stroke by a resilient member, vacuum draw, or return air pressure. This stroke is not generally used for compression purposes as in a conventional engine. Instead, the upper portion of the piston cylinder is vented during reciprocation so that the contents of the combustion chamber in the starting or rest position are at or near atmospheric pressure. This is primarily done because holding a compressed charge for what may be extended periods between cycles has not proven practical. However, as a result of the inherent thermal-to-mechanical output inefficiencies resulting from this lack of compression, the combustion chambers of intermittent linear motors are required to be fairly large for a given power output.
These relatively large uncompressed combustion chambers of intermittent linear motors, as well as being inherently inefficient, are especially sensitive to the presence of residual exhaust gases from previous cycles. Failure to remove such residual gases will result in a diluted charge and deterioration of burn speed, which is critical when driving a fastener. Thus, unless such gases can be substantially completely removed and replaced with a clean air/fuel mixture, subsequent cycles will deliver significantly less power.
It is, therefore, necessary to provide some type of efficient exhaust scavenging system in devices utilizing intermittent linear motors. Such systems should discharge exhaust gases from the tool as quickly as possible after combustion has been completed and useful work performed. This helps prevent the tool from overheating and can also minimize the amount of scavenging air required to completely clean out the remaining exhaust gases. There can be some variation due to the differing shapes and configurations of combustion chambers and their porting locations; however, it is generally necessary to pump clean air having a volume of at least 2.5 times the volume of the combustion chamber in order to adequately clean out (i.e. scavenge) exhaust gases prior to injecting fuel into the chamber. Representative prior art approaches the problem of rapidly and efficiently scavenging exhaust gases can be seen in U.S. Pat. Nos. 4,403,722; 4,712,379; and 4,759,318.
These patents generally rely on a temperature drop in the gases remaining in the combustion chamber after exhaust gases have been allowed to escape following a power stroke. This temperature drop forms a partial vacuum, causing scavenging air to be drawn in through check valves at the ignition end of the combustion chamber. A critical problem associated with these systems is the speed with which the scavenging operations of this type can be accomplished. As it takes time and temperature drop for a vacuum to be realized after the fastener has been driven, hot gases are allowed to stay in the tool for long periods of time up to 500 milliseconds. This causes the tool to heat up and lose power as well as severely limiting the operating speed of the tool.